This invention relates to a resonator and filter device. More particularly, the invention relates to a resonator that includes a microstrip line, which has an electrical length corresponding to a λ/2 wavelength, formed on a dielectric substrate, and to a filter obtained by provided a plurality of the resonators side by side on a dielectric substrate.
There is increasing activity toward the introduction of superconducting filters, which exhibit little loss in the pseudo-microwave band, for use in base stations for mobile communications. In general, the number of filter stages (number of resonators) must be enlarged in order to obtain a steep cut-off characteristic in filters for communication purposes. However, a problem which arises is a commensurate increase in pass band loss. Accordingly, the fact that a superconductor has a resistance that is two to three orders of magnitude lower than that of ordinary metal has become the focus of attention, and there is increasing introduction of superconducting filters adapted so as to minimize loss in the pass band by using a superconductor as the conductor of a filter. In particular, superconducting filters have in recent years become noteworthy as promising means for effectively utilizing frequency in mobile-band communications, increasing subscriber capacity and enlarging the area of base-station coverage.
YBCO (Y.Ba.Cu.O) having a critical temperature (Tc) on the order to 90 K is known as a superconducting material for superconducting filters. It is used at a Tc of 70 to 80 K, at which characteristics are stable.
FIG. 10 is a diagram illustrating the structure of a conventional radio-reception amplifying device equipped with a superconducting filter. A superconducting filter (SCF) 1 and a low-noise amplifier (LNA) 2 are secured on a cold head 4 and accommodated in a vacuum vessel 3. The cold head 4 is cooled by a freezer or refrigerator 5. The superconducting filter 1 and low-noise amplifier 2 are cooled by the freezer 5 via the cold head 4 and operate at Tc=70 K. The vacuum vessel 3 and freezer 5 are placed inside a case 6 so that they can be installed outdoors, terminals 7a, 7b and 8a, 8b provided on the case 6 and vacuum vessel 3 are connected by coaxial cables 9a, 9b, and terminal 7b→superconducting filter 1→low-noise amplifier 2→terminal 8b also are connected by a cable 9c. A receive signal 10 is input to the terminal 7a. 
As shown in (A) and (B) of FIG. 11, the superconducting filter 1 has a structure obtained by patterning, using YBCO film, filter electrodes 1b1, 1b2 and n stages (n=5 in the illustration) of λ/2 resonators 1c1 to 1c5 on an MgO substrate 1a of thickness t=0.5 mm, and sealing these in a package 1d made of an aluminum alloy as best seen in (A) of FIG. 11. The package 1d prevents leakage of electromagnetic field and cools the filter substrate 1a uniformly. In FIG. 11, (A) is a plan view in which an upper cover 1e (see (B) of FIG. 11) of the package has been removed, and (B) is a sectional view taken along line AA in (A). Further, reference characters 1f, 1g represent coaxial connectors and 1h (see (B) of FIG. 11) a ground plane formed by a YBCO film having a thickness of 0.4 μm.
In order to operate the superconducting filter at T=70 to 80 K, as mentioned above, the superconducting filter must be placed in the vacuum vessel, insulated from the outside and cooled using a refrigerator. To accomplish this, it is required that the filter be made small in size. Conventionally, use is made of a filter having a hairpin-shaped resonator structure formed by a microstrip line, as illustrated in (A) of FIG. 11. The hairpin filter has a simple resonator structure and a large number of prior art references have been published. The design is very simple and has become the basic structure of superconducting filters.
When such a hairpin filter, e.g., a hairpin filter (see FIG. 12) having a center frequency of 2 GHz, a bandwidth of 20 MHz and nine filter stages is designed, the size thereof is on the order of 525 mm2. More specifically, if the distance between hairpin resonators 1c1, 1c2, 1c3, 1c4, 1c5, 1c6, 1c7, 1c8, 1c9 is uniquely decided from filter design values and the resonators are disposed at this spacing, the dimensions of a single hairpin resonator are about
15 mm×2 mm vertically and horizontally. The dimensions of the overall filter and the occupied area are 15 mm×35 mm=525 mm2 vertically and horizontally. In FIG. 12. 1a denotes MgO substrate, 1b1 and 1b2 filter electrodes. 1d a package and 1g a coaxial connector.
In the superconducting hairpin filter, material constants vary and so do patterning precision in actuality. It is necessary to subject the resonator length of each individual resonator to trimming by a laser, adjust the resonance frequency of each oscillator and make an adjustment so as to obtain the desired filter characteristics. An example of a trimming method that can be mentioned is a method of trimming a superconducting filter by a laser in an operating temperature environment of low temperature.
Even if the superconducting hairpin filter is small in size, a plurality of filters are required simultaneously depending upon the communication system, and it is necessary that these be cooled by a single refrigerator. The insulated vacuum vessel becomes enormous, the overall receiving apparatus becomes large in size and of increased weight.
For example, in the 800-MHz band or 2-GHz band (IMT-2000), the base station apparatus requires two filters in one sector. In six sectors, that is a total of 12 filters required. Power consumption by the refrigerator is about 100 W per sector. If, by way of example, one refrigerator is used for every sector, about 600 W will be required for the six sectors, thereby necessitating several thousand watts of power consumption for the entire base station. Accordingly, cooling as many filters as possible simultaneously by one refrigerator is required in order to reduce power consumption by the overall base station and lower cost. Further, if filter area is large, there will be an increase in heat radiated from the vacuum vessel and an increase in power consumption by the refrigerator. For these reasons, it is desired that the filter be further reduced in size.
Further, if trimming is performed by a laser or the like, a very high machining precision is required conventionally. That is, a planar-circuit-type filter forming a pattern on a substrate is such that even if pattern formation is performed accurately by carrying out etching in accordance with the design pattern, the oscillation frequencies of each of the resonators will differ from the design values due to variations in specific inductivity of the dielectric substrate and unevenness of the substrate. Accordingly, the pattern of the resonator is formed somewhat long and the desired resonance frequency is adjusted by cutting off the resonator end REP (see FIG. 12) using a laser or the like while the resonance frequency of each resonator is measured by a probe or the like. This is carried out for all of the resonators. However, this task relies upon human intervention and must be performed with precision. For these reasons, a structure having high redundancy with regard to trimming, i.e., a filter that exhibits little change in characteristics with regard to trimming, is desired.